1. Field of the Invention
The present invention relates to a semiconductor laser-diode module for use in optical transmission. It is particularly useful for a semiconductor laser-diode module intended for super high-speed operation at 10 Gbit/s or higher.
2. Related Art
In recent years, speed of optical transmission has been increased rapidly and an optical transmission system at 10 Gbit/s has already been put to practical use. In existent 10 Gbit/s systems, an electro-absorption (EA) type modulator with an integrated laser-diode (hereinafter simply referred to as LD) for generating signals at 10 Gbit/shave been used. The EA modulator with an integrated LD generates optical signals by turning continuous wave light generated from the LD to ON and OFF. The module for the EA modulator with integrated LD is disclosed, for example, in Japanese Patent Laid-open No. 9-252164. In this module, since the range of operation temperature of the EA modulator is narrow, it is necessary that a Peltier device (a the
o-electric cooler) be incorporated to regulate the LD device at a constant temperature. This results in a problem that the module is expensive and the power consumption is large.
While on the other hand, the high speed modulation characteristics and high temperature characteristic of LOs have been improved remarkably recently, and optical signals at 10 Gbit/s can be generated directly by ON and OFF of current supplied to LDs. The direct LO modulation method provides a wide operation temperature range and does not require temperature control by the Peltier device. Accordingly, the module can be reduced in size and cost and the power consumption can be decreased outstandingly. To put the direct modulation method to practical use requires an LD module having a mechanism capable of transmitting driving signals at 10 Gbit/s to an LO device at reduced loss and reflection. LD modules performing high-speed modulation at the level of 2.5 Gbit/s have already been put to practical use. FIG. 19 shows an example of an equivalent circuit model in a state where an existent LD module 1 or package and a driver IC 8 for driving the LD module are connectedly mounted on a printed circuited board. The LD module has a pin 2 for inputting driving signals. Since the pin has stray inductance, it is represented by a symbol showing inductance. Further, the LD module has therein a high-frequency transmission line 3 under impedance matching and a terminating resistor 4 connected to the end of the line. The terminating resistor 4 and the LO device 6 are connected by a wire bond 5. That is, the terminating resistor 4 is connected in series with the LD device 6. Driving signals (electric signals) generated from a driver IC 8 are supplied through the high-frequency transmission line 9 under impedance matching disposed on the printed circuit board, the pin 2 of the LD module, the high-frequency transmission line 3 in the package 1, the terminating resistor 4, and the wire bond 5 to the LD device 6. Reflection of the driving signals is decreased by setting the value of the terminating resistance such that the sum of the resistance value of the terminating resistor 4 and the resistance value (usually of about 5 xcexa9) of the LD 6 matched with the impedance of high-frequency transmission lines 3 and 9. In this case, the impedance of the high-frequency transmission line is usually set to 20 to 30 xcexa9) in view of the consumption power of the driving IC and easy constitution. However, since the pin 2 and the wire bond 5 have a stray inductance component (about 2 to 3 nH as the sum of them), electric signals are reflected at the pin 2 and the wire bond 5 of the module and returned to the driver IC 8 also for the electric signals at about 2.5 Gbps/s. The state of signal reflection is shown by arrows in FIG. 19.
The signals returned to the driver IC are reflected at the driver, enter the LD module and transmit to the LD device again. This generates ringing (ghost) in the optical signal waveform of the LD module to deteriorate the optical waveform.
Means for solving this problem is described, for example, in Japanese Patent Laid-open No. 5-327617. This example is shown in FIG. 20. This is a method of inserting a damping resistor 10 between the terminal end of the high-frequency transmission line 9 on the printed circuit board and the ground, that is, a method of inserting the damping resistance 10 so as to be in parallel with the LD module. In FIG. 20, since reference numerals identical to those in FIG. 19 indicate identical members, detailed explanations thereof will be omitted. When the parallel damping resistance 10 is inserted, it is necessary to increase the IC driving current by so much as the current flowing to the damping resistor 10. However, since the electric signals reflected at the input pin 2 or the wire bond 5 are released by way of the resistor 10 to the ground, deterioration of the waveform caused by reflection can be decreased.
However, when the speed of the driving signals increases in the existent LD module as far as about 10 Gbit/s, since reflection by the stray inductance component of the pin or the wire bond increases more and more, there is a problem that the degradation of the wave is further increased. In view of the above, even when the parallel damping resistances disposed on the printed circuit board, for example, in the same manner as in FIG. 20, there is still a problem that the degradation of the waveform due to the reflection of the driving signal cannot be decreased for the signal at 10 Gbit/s.
An object of the present invention is to provide a laser-diode module with less ringing due to reflection of electric signals even when modulation at super high-speed of generally 10 Gbit/s or more and capable of generating good optical waveforms.
The laser-diode module can be provided by the following means.
A pin for inputting a driving signal is disposed at a package of a laser-diode module (i.e., a terminal for inputting an electric signal), and a laser-diode device, a high-frequency transmission line and a terminating resistor (i.e., a first resistor) thereof and a parallel damping resistor (i.e., a second resistor) are disposed in the inside of the package. One end of the high-frequency transmission line is connected with the pin for inputting a driving signal. The high-frequency transmission line and the terminating resistor thereof are connected in series with the laser-diode device. On the other hand, the parallel damping resistor is connected in parallel with the laser-diode device. In the invention, it is particularly important to place the laser-diode device, the high-frequency transmission line, the terminating resistor thereof (i.e., the first resistor) and a parallel damping resistor (that is second resistor) in the package. In a specific example, a first electrode of the laser-diode device is connected by way of the terminating resistor to the other end of the high-frequency transmission line and the parallel damping resistor is disposed between the connection point of the high-frequency transmission line and the terminating resistor, and the second electrode of the laser-diode. To obtain a good optical waveform, it is desirable to set the distance between the parallel damping resistor and the laser-diode to 2.4 mm or less. To reduce the consumption power by a bias current, the module may be constituted such that a second pin for applying a DC bias current is disposed at the module and is connected to the first electrode of the laser-diode device by way of a device for blocking the high frequency signal (for example, inductor or ferrite bead). In a case where the impedance of the high-frequency transmission lines is from 20 to 30 xcexa9, the resistance value of the parallel damping resistor device is set preferably to 67 to 300 xcexa9.